Roy Spicer, a volunteer with the Sudbury Swanwatch group, also expressed his disappointment that the police do not believe a crime was committed.

Mr Spicer said a dog off the lead went into the river near Brundon Hall Farm on Monday and attacked a swan, killing it.

While he did not see the attack itself, he said he had been informed by a witness.

In December this newspaper reported seven swans in the past year had died as a result of attacks by dogs according to Mr Spicer.

Following this latest attack he said dog owners needed to take greater responsibility for their dogs, adding: “It’s not many dog owners. It’s a very small proportion.

“Most dog owners are really responsible and put their dog on a lead when they notice there’s wildlife about.”

Mr Spicer, who has taken to carrying a video camera on his rounds, said under the Wildlife and Countryside Act 1981 Monday’s incident was indeed a crime.

But a Suffolk police spokeswoman said: “Police were called around 5.30pm on Monday to Melford Road to reports that a swan had died. Enquiries were made to establish the circumstances around the incident and it was concluded that no criminal offences had taken place.”

A spokeswoman for the Royal Society for the Prevention of Cruelty to Animals (RSPCA) said legally dog owners should have charge and control of their animals in a public place.

“Swans and other wildlife are protected by law and you could face prosecution if your dog attacks and kills. It is important for dog owners to remember they are ultimately responsible for what their pet does so if walking in a park or in the countryside near livestock dogs are best kept on a lead.

“We would like to remind people that swans are protected under the Wildlife and Countryside Act 1981 and anyone found guilty of disturbing, injuring or killing them could face a fine of up to £5,000 and a six-month prison sentence.”

Mr Spicer said when people spotted his video camera they made sure their dog was on a lead, and he added a new fence at Brundon had reduced the number of attacks.